Photoreaction Pathways of Bacteriorhodopsin and Its D96N Mutant as Revealed by in Situ Photoirradiation Solid-State NMR

Bacteriorhodopsin (BR) functions as a light-driven proton pump that transitions between different states during the photocycle, such as all-trans (AT; BR568) and 13-cis, 15-syn (CS; BR548) state and K, L, M-1, M-2, N, and O intermediates. In this study, we used in situ photoirradiation C-13 solid-state NMR to observe a variety of photo-intermediates and photoreaction pathways in [20-C-13]retinal-WT-BR and its mutant [20-C-13, 14-C-13]retinal-D96N-BR. In WT-BR, the CS state converted to the CS* intermediate under photoirradiation with green light at -20 degrees C and consequently converted to the AT state in the dark. The AT state converted to the N intermediate under irradiation with green light. In D96N-BR, the CS state was converted to the CS* intermediate at -30 degrees C and consequently converted to the AT state. Simultaneously, the AT state converted to the M and L intermediates under green light illumination at -30 degrees C and subsequently converted to the AT state in the dark. The M intermediate was directly excited to the AT state by UV light illumination. We demonstrated that short-lived photo-intermediates could be observed in a stationary state using in situ photoirradiation solid-state NMR spectroscopy for WT-BR and D96N-BR, enabling insight into the light-driven proton pump activity of BR.